<html><head>
<meta name="viewport" content="width=device-width">
<style>
.d1 {position:fixed; top:5; right:5; z-index:2; overflow:hidden;}
.o {background:green; height:40px; width:200px;}
.t { width:2000px; height:198px; background-color: lightgray; border: 1px solid blue;}
body { margin: 0px; }
</style>
<script>

var fixed;

function remove_child()
{
  fixed = document.getElementById("d1");
  fixed.parentElement.removeChild(fixed);
}

function add_child()
{
  document.body.appendChild(fixed);
}

</script>
</head>
<body>
<div class="d1" id="d1"><div class="o">This is a test</div></div>
<div class="t">
000
</div>
<div class="t">
200<br>
<button onclick="remove_child();">remove fixed</button>
</div>
<div class="t">
400<br>
<button onclick="add_child();">add fixed</button>
</div>
<div class="t">
600<br>
</div>
<div class="t">
800
</div>
<div class="t">
1000
</div>
<div class="t">
1200
</div>
<div class="t">
1400
</div>
<div class="t">
1600
</div>
<div class="t">
1800
</div>
<div class="t">
2000
</div>
<div class="t">
2200
</div>
<div class="t">
2400
</div>
<div class="t">
2600
</div>
<div class="t">
2800
</div>
<div class="t">
3000
</div>
<div class="t">
3200
</div>
<div class="t">
3400
</div>
<div class="t">
3600
</div>
<div class="t">
3800
</div>
<div class="t">
4000
</div>
</body></html>
